In large endless non-metallic conveyor belts, which may have metal reinforcing cables therein, used to convey bulk material, there is a possibility of encountering a rip in the belt, for example, by a sharp object dropped thereon at the loading station. It is desirable promptly to detect such rips and, preferably, to shut down the conveyor belt upon such detection, thereby minimizing damage to the belt. One such conveyor belt rip detector is disclosed in U.S. Pat. No. 3,792,459. In such rip detector plural antennas, which may be single electrical conductors, are embedded in the belt transversely to its length at spaced-apart locations in the belt. An electrical signal is coupled by respective antennas from a transmitter to a receiver as the belt moves and the respective antennas pass in capacitive coupling relation with the transmitter and receiver at a rip detector station, and the receiver thus delivers an input signal to detector circuitry which interprets the same as an indication of satisfactory belt integrity. However, a broken antenna, for example at a place where the belt has been ripped, will not couple the transmitter signal through to the receiver, and the detector then senses the same as an indication of the occurrence of a rip condition. The detector circuit in such patent is operative after the lapse of a predetermined time period corresponding to the passage of a given number of broken antennas past the rip detector station to produce a distinguishable output that activities an alarm and/or deactivates the conveyor belt drive.
A number of improvements for use in conveyor belt rip detectors are disclosed in U.S. Pat. No. 4,228,513, the disclosure of which is hereby incorporated by reference. One of such improvements includes a means for detecting the progress of the conveyor belt to know when an antenna should be at the rip detector station. If there is no antenna present then, a prompt shutdown of the conveyor belt drive may be effected.
Wear, stretching, contraction, dirt, other environmental conditions, etc. may cause a variation in the efficiency of signal coupling, whether of the capacitive, inductive, optical, or any other type of coupling, between the antennas (or other signal coupling means carried by the belt) and the transmitter and the receiver at a rip detector station. Such efficiency variation will vary the magnitude or other parameter of the input signal delivered from the receiver to the detector, which may detrimentally affect operation of the entire system.
Thus, it would be desirable to provide signal information to the detector at a relatively accurately controlled magnitude (or other parameter). In copending, commonly assigned U.S. patent application Ser. No. 126,218, filed Mar. 3, 1980, the disclosure of which is hereby incorporated by reference, such magnitude control is effected digitally, step-wise and efficiently.
It has been discovered that in some operational environments of conveyor belt rip detectors the electrical noise is so great that the poor signal to noise ratio significantly decreases the accuracy of the rip detector function. Moreover, and very importantly, it has been discovered that in a conveyor belt that carries antennas intended for capacitive coupling with the transmitter and receiver at a rip detector station, much of the electrical noise introduced to the receiver is derived from cross coupling, primarily of a capacitive nature, between the capacitive probes of the transmitter and receiver via the belt itself and/or apparatus associated therewith, such as the rollers, drive wheels, support frame, etc. In connection with such discovery, an equivalent electric circuit model of the transmitter/belt/receiver connection has been developed and used to determine an efficient technique for improving signal to noise ratio even in extremely electrically noisy environments.